Various compounds are used as sealants to achieve seals at conjoined substrate surfaces. Such surfaces requiring sealants can exist in the interior of the aircraft and include, for example, areas within an aircraft or spacecraft wing, including, without limitation, the fasteners on the interior of fuel tanks. Such sealant materials comprise various characteristics beneficial to their contemplated end use. With respect to the use of sealants on aircraft and spacecraft, additional characteristics come into play, including, for example, the overall weight that sealants may add to an aircraft. Such additional weight can impact fuel consumption, aircraft performance, effective range, cost, etc.
Many sealant formation processes in the aerospace industry implement polysulfide sealants that undergo a gas-forming chemical reaction to expand and otherwise “foam” the sealant. Efforts to foam such sealants have used hydrides in combination with polysulfide compounds. However, the hydrogen gas that is liberated from the reaction is dangerous from an explosion and/or toxicity risk, especially in an enclosed environment where many sealing operations occur.
Polysulfide polymers in liquid form are viscous fluids than can be converted to solid form at room temperature with the addition of curing agents. When blowing agents are added to the polysulfide polymers and the curing agent, a cellular “foamed” rubber product results. Liquid polysulfide polymers are cured by converting the thiol terminal group to create disulfide bonds that link short chain segments of the liquid polymer to long chain segments resulting in a cured polymer with elastomeric properties. Lead peroxide and cumene hydroperoxide have been suggested as useful curatives for polysulfide reactions. However, the toxicity of the reaction byproducts remains a concern.
Efforts have been made to reduce the amounts of sealants used in aircraft manufacturing. Some efforts have focused on the reduction of the density of sealants used in the production and manufacture of aircraft and spacecraft. For example, known methods for reducing sealant density include introducing micro-structures, such as microballoons, to the sealant formulation. The thin-walled micro-structures may or may not be air-filled. Such structures, when introduced into the sealant formulation reduce the density of sealants. In essence, the presence of the micro-structures in the sealant formulation increases the overall volume of the sealant while keeping the weight of the sealant applied in such volume substantially constant. The introduction of physical micro-structures has, however, proven problematic as micro-structures can be susceptible to collapse or rupture when ambient pressure is varied such as, for example, the pressure changes prevalent during flight, especially high altitude flight.